1. Field of the Invention
The present invention relates to a die clamping unit of an injection-molding machine, and more specifically, to the structure of an electric direct-acting die clamping unit directly transmitting thrust from a driving source to a movable platen without using a toggle mechanism.
2. Description of the Related Art
FIG. 6 shows a schematic structure of a conventional electric direct-acting die clamping unit.
A stationary platen 3 and a back plate 62 are arranged at both ends of a base 1 so as to face each other. A movable platen 4 is arranged in the front of the back plate 62 so as to face the stationary platen 3. A stationary die 5 is attached to the front surface of the stationary platen 3, whereas a movable die 6 is attached to the front surface of the movable platen 4.
The back plate 62 and the stationary platen 3 are connected by four tie rods 67. A through-hole is formed at each of the four corners of the movable platen 4. The four tie rods 67 are passed through the corresponding through-holes. The movable platen 4 slidably moves back and forth along the tie rods 67, thereby clamping and unclamping the dies.
The movable platen 4 is connected to the front surface of the back plate 62 via a ball screw 70. More specifically, a load cell 75 is fixed to the back surface of the movable platen 4. The tip portion of the threaded rod 71 of the ball screw is fixed to the back surface of the load cell 75. The nut 72 of the ball screw is rotatably supported by the back plate 62 via a bearing 77.
A motor 80 is arranged under the base 1. A pulley 82 is attached to the end surface (at the side close to the movable die) of the nut 72. Another pulley 81 is attached to the shaft of the motor 80. A timing belt 83 goes around the two pulleys 81 and 82. When the motor 80 rotates the nut 72, the threaded rod 71 moves back and forth along its axis, thereby moving the movable platen 4 back and forth along the tie rods 67.
As mentioned above, in the conventional electric direct-acting die clamping unit, the movable platen 4 is moved by the ball screw 70 to clamp and unclamp the dies.
(A Problem of Conventional Direct-Acting Die Clamping Unit)
When the dies are clamped by the conventional direct-acting die clamping unit, the reaction force is received only by four tie rods 67. The tie rods 67 are rigid since the total sectional area of the tie rods is large. Therefore, even if the reaction force is applied to the tie rods, the tie rods do not stretch so much. Therefore, the die clamping force (i.e., compressive stress applied to the die surfaces) sharply increases immediately after the die surface of the stationary die 5 comes into contact with that of the movable die 6. As a result, the overshoot of the die clamping force occurs. Therefore, it becomes difficult to control the clamping force accurately.
Furthermore, since an excessively large load is applied to structural parts of the die-clamping unit, the life of parts are short.